Lead solder-free electronics

ABSTRACT

A composition may have metal nanoparticles having a diameter of 20 nanometers or less and have a fusion temperature of less than about 220° C. A method of fabricating the metal nanoparticles may include preparing a solvent, adding a precursor with a metal to the solvent, adding a first surfactant, mixing in a reducing agent, and adding in a second surfactant to stop nanoparticle formation. Copper and/or aluminum nanoparticle compositions formed may be used for lead-free soldering of electronic components to circuit boards. A composition may include nanoparticles, which may have a copper nanocore, an amorphous aluminum shell and an organic surfactant coating. A composition may have copper or aluminum nanoparticles. About 30-50% of the copper or aluminum nanoparticles may have a diameter of 20 nanometers or less, and the remaining 70-50% of the copper or aluminum nanoparticles may have a diameter greater than 20 nanometers.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims the benefit of priority under 35 U.S.C.§119 from U.S. Provisional Patent Application Ser. No. 61/097,175,entitled “SOLDER-FREE ELECTRONICS,” filed on Sep. 15, 2008, which ishereby incorporated by reference in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with Government support, and the Government hascertain rights in the invention under Contract No. FA8650-08-C-5509awarded by AFRL, Wright-Patterson Air Force Base, OH 45433.

BACKGROUND

Until recently, most electronic circuits were manufactured usinglead-based soldering material. However, in response to regulatorychanges, lead (Pb) is being phased out of electronic manufacturing. Forexample, under the Restriction of Hazardous Substances (RoHS) directive,the European Union outlawed the use of Pb for most electronics producedafter June 2006. Military and medical products may remain exempt forsome time, but will eventually be subject to similar restrictions.Electronics made using lead-based soldering material is reliable and alarge investment has gone into the manufacturing infrastructure thatuses lead-based material for soldering electronic components. Theworldwide phasing out lead-based soldering material has raised seriousconcerns regarding reliability of alternative methods and also raisedissues of additional capital investment required to phase out certainmanufacturing infrastructure.

Lead-free solder replacements in practice today use eutectic alloys suchas tin-copper (SnCu), tin-silver (SnAg), and tin-copper-silver (SnAgCu),all generally referred to as Sn—Ag—Cu (SAC) material. The use of SACsoldering materials suffer from several drawbacks. For example, whilelead-based soldering material has a eutectic at 183° C., SAC solderingmaterials have higher melting points with processing temperatures around260-300° C. To withstand such high temperatures experienced during thesoldering process, other materials in an electronic product, such as aprinted circuit board (PCB) or a component packaging material isrequired to be more robust and are therefore typically more expensive.For example, glass transition temperature (temperature at which amaterial ceases to be solid and becomes flexible) of several commonlyavailable polymers are: FR-4 (<130° C.), BT epoxy (180° C.), cyanateester (230° C.) and polyimide (250-250° C.). The cost of the polymerswith the higher glass transition temperatures tends to be more. Thetypical PCBs are of the FR-4 type or polymer substrate materials thathave even a lower glass transition temperature than that of FR-4. In oneaspect, the cost of the polymers with higher glass transitiontemperature is an important issue. FR-4, at glass transitiontemperatures higher than 125° C., can withstand a process temperature upto 230° C., which has been the proven real-world performance.

Furthermore, reliability of electronics products manufactured using SACsoldering material suffers due to uncontrolled tin whisker growthresulting in rapid and uncontrollable joint failure. The reliabilityissue is most critical for military and space applications where longlife and reliability is of utmost importance.

In one aspect, a better technique for lead solder-free electronics isneeded.

SUMMARY

In one exemplary aspect, a composition may comprise metal nanoparticleshaving a diameter of 20 nanometers or less but greater than zero. Themetal nanoparticles may comprise a copper or aluminum nanocores. Thecomposition may have a fusion temperature of less than about 220° C.

In another exemplary aspect, a method of fabricating a nanomaterial isdisclosed. The method may comprise preparing a solvent and adding aprecursor to form a solution. The precursor may comprise a metal, whichmay be aluminum or copper. The method may further comprise adding afirst surfactant to the solution, mixing in a reducing agent, at amixing rate, with the solution until symptoms of nanoparticle formationoccur in the solution, and adding a second surfactant to the solution tostop nanoparticle formation and agglomeration after the symptoms ofnanoparticle formation occur.

In another exemplary aspect, a method of soldering an electroniccomponent to a circuit board is disclosed. The method may compriseapplying a soldering paste, which comprises a nanomaterial, between theelectronic component and the circuit board. The nanomaterial maycomprise a plurality of nanoparticles surrounded by a surfactant. Themethod may further comprise heating the soldering paste until thesurfactant melts causing the plurality of nanoparticles to fuse, andallowing the heated soldering paste to cool down to form a solderingjoint between the electronic component to the circuit board.

In another exemplary aspect, a composition may comprise nanoparticles,which may comprise a copper nanocore, an amorphous aluminum shell and anorganic surfactant coating. The nanoparticles may have a diameter of 20nanometers or less but greater than zero.

In another exemplary aspect, a method of manufacturing a printed circuitmay comprise applying a metal nanomaterial, which may comprise asurfactant, to a surface. The method may further comprise tracing, by aheat source, a circuit line of the printed circuit on the surface. Thetracing may cause the surfactant on the traced circuit line toevaporate. The method may further comprise treating the surface with asolvent to cause the untraced metal nanomaterial to wash off.

In yet another exemplary aspect, a composition may comprise copper oraluminum nanoparticles. About 30-50% of the copper or aluminumnanoparticles may have a diameter of 20 nanometers or less but greaterthan zero, and the remaining 70-50% of the copper or aluminumnanoparticles may have a diameter greater than 20 nanometers.

It is understood that other configurations of the subject technologywill become readily apparent to those skilled in the art from thefollowing detailed description, wherein various configurations of thesubject technology are shown and described by way of illustration. Aswill be realized, the subject technology is capable of other anddifferent configurations and its several details are capable ofmodification in various other respects, all without departing from thescope of the subject technology. Accordingly, the drawings and detaileddescription are to be regarded as illustrative in nature and not asrestrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating an example of an operation offabricating nanomaterial, in accordance with certain configurations ofthe present disclosure.

FIG. 2 is a conceptual illustration of a nanoparticle, in accordancewith certain configurations of the present disclosure.

FIG. 3A is a conceptual block diagram illustrating application of asoldering nanomaterial to solder an electronic component to a circuitboard, in accordance with certain configurations of the presentdisclosure.

FIG. 3B is a conceptual block diagram illustrating the solderingnanomaterial in FIG. 3A, after application of heating, in accordancewith certain configurations of the present disclosure.

FIG. 3C is a conceptual block diagram of a soldering joint formed fromthe soldering nanomaterial in FIGS. 3A and 3B, in accordance withcertain configurations of the present disclosure.

FIG. 4 is a flow chart illustrating an example of a sequence ofoperations for soldering an electronic component to a circuit board, inaccordance with certain configurations of the present disclosure.

DETAILED DESCRIPTION

The detailed description set forth below is intended as a description ofvarious configurations of the subject technology and is not intended torepresent the only configurations in which the subject technology may bepracticed. The appended drawings are incorporated herein and constitutea part of the detailed description. The detailed description includesspecific details for the purpose of providing a thorough understandingof the subject technology. However, it will be apparent to those skilledin the art that the subject technology may be practiced without thesespecific details. In some instances, well-known structures andcomponents are shown in block diagram form in order to avoid obscuringthe concepts of the subject technology. Like components are labeled withidentical element numbers for ease of understanding.

U.S. patent application Ser. No. 11/798,529, entitled “RHENIUMNANOPARTICLES” is incorporated herein by reference in its entirety. U.S.patent application Ser. No. 12/426,160, entitled “CARBON NANOTUBESYNTHESIS USING REFRACTORY METAL NANOPARTICLES AND MANUFACTURE OFREFRACTORY METAL NANOPARTICLES” is incorporated herein by reference inits entirety.

In one aspect, the desired fusion temperature for the solder is achievedusing a property of certain nanostructured materials that their fusiontemperatures drop with reduction in particle size. The term “fusiontemperature” may refer to the temperature at which a soldering materialliquefies, giving the appearance of melting. For example, rheniumnanoparticles having between 5 and 25 nanometer (nm) diameter exhibit afusion temperature of about 600-800° C., a significant reduction from3180° C., the melting point of non-nanoparticled rhenium. Similarly,copper also exhibits a similar property in that the fusion temperatureof copper nanoparticles (or copper nanomaterial) is significantly lowerin comparison to the melting point of non-nanoparticle copper (1083°C.). Aluminum nanomaterials also exhibit such a reduction in fusiontemperature. Generally speaking, lower diameter nanoparticles have lowerfusion temperature. For example, the fusion temperature of a coppernanomaterial, having a nanoparticle size in the 2-5 nanometer range,could have a fusion temperature of approximately 200° C., whereas withthe particle size in the 40-50 nm range, the copper nanomaterial mayhave a fusion temperature of about 750° C., and at 20-30 nm particlesize, the fusion temperature may be about 450° C. Similarly, ananoparticle mixture comprising nanoparticles sized 20 nm or smaller mayhave a fusion temperature of less than about 220° C.

It is advantageous to use a material such as copper or aluminum toproduce low fusion temperature nanomaterial for use as a soldering agentbecause of high conductivity of copper or aluminum. Furthermore, it ispossible to lay down these metals on circuit boards as thin traces or inform of ball grid arrays (BGA) having very fine pitch, therebyfacilitating manufacturing of densely packed circuitry. Table 1 showselectrical and thermal properties of copper compared to tin-based andlead-based soldering materials.

TABLE 1 Oxide Sn Solder/ Pb Solder/ Material free Cu 3.5% Ag 5% SnUltimate Tensile Strength (Mpa) 221-455 38.7 27.6 Electrical resistivity(ohm-cm) 1.71E−06 1.10E−05 2.00E−05 Thermal conductivity (W/m-K) 383-39133 23

FIG. 1 is a flow chart showing operations performed in fabricating ananomaterial, in accordance with certain configurations of the presentdisclosure. In certain configurations, the operations shown in FIG. 1may be performed in a single flask. The flask may be, for example, asdisclosed in the above referenced U.S. patent application Ser. No.12/426,160. In the description below with respect to FIG. 1, processesare generally described with reference to copper nanoparticlefabrication. However, it is understood that nanoparticles for othermetals (e.g., aluminum) may be fabricated along the same principles.

At operation 102, a solvent is prepared. The solvent acts as a medium inwhich the subsequent nanoparticle fabrication takes place by a reductionprocess in a solution comprising suitable precursors, as described inthe present disclosure. Some examples of solvents are disclosed in theabove referenced U.S. patent application Ser. No. 11/798,529.

At operation 104, one or more copper precursors are added to the solventto form a solution. A precursor provides the solvent with copper atomsnecessary to form copper nanoparticles. Examples of copper precursorsinclude copper chloride (CuCl₂), copper oxide (CuO) by dissolution inacid or base with the help of amines (e.g., as described in U.S. Pat.No. 3,637,508), or copper hydroxide (Cu(OH)₂) with ammonia. In certainconfigurations, copper nanoparticles may be made via the reduction of asuitable copper precursor (salt) such as halide, nitrate, acetate,sulphate, formate and oxide, etc.

In certain configurations, a copper precursor requires drying to avoidoxidization of nanoparticles by moisture and air-oxygen. In certainconfigurations, in situ drying may be performed. For example, hydrouscopper chloride (CuCl₂*2 H₂O) may be dried using processes that may berepresented by the following chemical equations (1) and (2).

CuCl₂*2H₂O+2HC(OCH₃)₃→CuCl₂+2HC(O)OCH₃+4CH₃OH  (1)

CuCl₂*2H₂+2(CH₃)₂C(OCH₃)₃→CuCl₂+2CH₃C(O)CH₃+4CH₃OH  (2)

One skilled in the art will recognize that the chemical reaction aboveproduces anhydrous CuCl₂ and, as a side product, a desired anhydroussolvent methanol, thus eliminating time-consuming drying procedures.

At operation 106, a first surfactant is added to the solution. Theaddition of the first surfactant may help control the size ofnanoparticles by controlling the range and distribution of diametersizes of the nanoparticles. In certain configurations, the firstsurfactant may be chosen from a variety of organic amines having variouschain lengths. For copper nanomaterial formation, the amines are chosen,in part, because the amines coordinate well to copper, but at the sametime can be removed easily upon moderate heating or vacuum to expose thenanoparticles enabling them to fuse.

At operation 108, a reducing agent is mixed in with the solution at apredetermined mixing rate. The reducing agents may include one or moreof lithium (Li), sodium (Na) or potassium (K) with a suitable catalyst.In certain configurations, Sodium Borohydride (NaBH₄) may be used as areducing agent. The chemical reaction for formation of nanoparticles,represented as nanoCu, when using copper chloride precursor and sodiumion as the reducing agent, is shown in Eq. (3)

CuCl₂+2Na*→2NaCl+nanoCu  (3)

In certain configurations of the present disclosure, copper oxide (CuO)may be used as the precursor, dissolved in a mineral acid or base andcitric acid, ascorbic acid or sodium borohydride used as the reducingagent. The chemical reaction when using copper oxide may be representedby equations (4a) and (4b) below.

CuO+2HCl→2Cl⁻+Cu²⁺+H₂O  (4a)

Cu²⁺+ascorbic acid→nanoCu+dehydroascorbic acid+2H⁺  (4b)

In certain configurations, copper chloride (CuCl₂) may be used as theprecursor. When using copper chloride with sodium borohydride as thereducing agent, the corresponding reaction maybe represented by equation(5) below.

CuCl₂+2NaBH₄→nanoCu+2NaCl+H₂+B₂H₆  (5)

In certain configurations, about 3 grams of CuCl₂*2H₂O may be dissolvedin about 45 ml of glyme or tetrahydrofuran (THF) with about 10 mldihexylamine and about 4 grams DDA. Sodium borohydride (about 19 ml 2Msolution) may be added at a controlled rate at room temperature. Whencolor of the solvent turns to green, about 30 ml water with 5-10%diethylenetriamine (DETA) may be added and 30 ml hexane with 5-10%n-butylamine may be mixed by stirring for a few minutes. Thenanoparticles are hydrophobic and may transfer into the organic phaseand may be extracted and separated from the byproducts in such afashion. The hexane may be evaporated to isolate the solidnanoparticles. In certain configurations, larger nanoparticles may beseparated by centrifugation.

In certain configurations, copper nitrate may be used as the precursorand citric or ascorbic acid may be used as the reducing agent. Theresulting chemical reaction may be represented by equation (6) below.

Cu(NO₃)₂+citric acid/ascorbic acid→nanoCu+organic side products  (6)

In certain configurations, the following steps may be used to performthe operations.

(1) Provide a 50 mL aqueous solution with a concentration ofapproximately 0.4 M L-ascorbic acid containing 4 grams PVP (solution 1).

(2) Provide a 50 mL aqueous solution of 1 g Copper (II) acetate×1 H₂Oand 1 ml ammonia water (conc) and 7 ml n-butylamine and 4 g PVPresulting in a blue solution (Cu²⁺-ammonia complex) (solution 2).

(3) Add solution 2 to solution 1,

(4) Stir and heat to +65° C. (about 30 min duration)

(5) Allow the mixture to cool back to approximately room temperature.

(6) Add, using a controlled rate, about 25 drops of sodium borohydride(2M solution) until color of the mixture starts to change.

(7) Extract nanoparticles by a process such as centrifugation orextraction with hexane as outlined above.

Use of reducing agents that have a relatively high (e.g., havingmagnitude >2 volts) reduction potential helps in formulation ofnanoparticles from electropositive elements. Accordingly, in one aspect,the reducing agent comprises a molecular sodium solution formed in thepresence of naphthalene with approximately 2.50 volts reductionpotential.

At step 110, the solution is monitored for symptoms of formation ofnanoparticles. The symptoms may include change of color of the solutionand/or amount of gas being released (bubbling) during the process. Forexample, when using CuO as the precursor for the formation of coppernanomaterial, the color of the solution initially is blue-green. Afteradding the reducing agent, as the reaction proceeds the color changes todark green indicating formation of nanoparticles followed by yellow andorange-brown, when the nanoparticles begin to grow to larger sizes.Accordingly, in certain configurations, the mixing rate of the reducingagent is held sufficiently low (e.g., 1.5 milliliter per minute) suchthat the reducing agent is mixed in only while the symptoms ofnanoparticle formation of the desired size are exhibited. When thesymptoms change (e.g., yellow color begins to turn to brown), the mixingof reducing agent may be discontinued.

In certain configurations where NaBH₄ is used as the reducing agent(e.g., equation (5) above), during the formation of nanoparticles gasmay be released (H₂ and B₂H₆) and may be a visible indication that thenanoparticle formation process is occurring.

At step 112, when the symptoms of formation of nanoparticles change(e.g., indicating that nanoparticles of a certain size have formed inthe solution), a second surfactant may be added to the solution tofacilitate termination of nanoparticle formation or agglomeration. Thesecond surfactant may also help add stability to the nanoparticlesformed in the solution by protecting the nanoparticles from furtherexposure to reacting chemicals and growth. Stabilization and protectionof such nanoparticles help stop the nanoparticles from oxidizing andreacting with chemicals in the air when exposed to air oxygen andmoisture. Oxidation and chemical reactions are undesirable becauseexposed nanoparticles can exhibit pyrophoric behavior.

In certain configurations of the present disclosure, organic amines,phosphines, and sulphur containing organic ligands may be used as thesecond surfactant. The choice of a second surfactant may involve atrade-off between the boiling temperature of a surfactant (thetemperature at which the surfactant allows encapsulated nanoparticles tobecome relatively free) and the predictability of size of nanoparticles.For soldering, a surfactant that facilitates the fusion process duringsoldering by releasing metal nanoparticle for joining and fusion may bedesirables. Accordingly, in one aspect, amines, known to be releasableupon moderate heating, maybe used as the second surfactants. Amines areknown to stabilize highly reactive alane (AlH₃) and such compounds arecommercially available (e.g., (CH₃)₃N—AlH₃, (C₂H₅)₃N—AlH₃). Other aminespossible for use as the second surfactant include n-butyl, hexyl, octyl,dodecyl amine, tripentyl amine, triethylenediamine and ethylene diamine.In certain configurations, phosphines such as tributylphosphine andtrioctylphosphine as well as trioctylphosphineoxide may also be used assecond surfactants or in combination with an amine. Generally speaking,a phosphine surfactants may exhibit a higher boiling point and may bondmore strongly with the nanoparticles and therefore volatilize lessreadily. In addition, quaternary ionic amines such as tetraoctylammoniumbromide and tetraethylammonium bromide may also be used to controlnanoparticle size in the early stages of particle growth by covering thenanoparticle surface (weak electrostatic forces) and slowing downagglomeration. In certain configurations, a mixture of surfactantshaving weaker and stronger bonding characteristics may be used toachieve a balance between the fusion temperature of the resultingnanomaterial and how quickly the process of nanoparticle formation needsto be stopped.

At step 114, the resulting solution comprising nanoparticle material maybe changed to a form suitable for storage or use in another application.For example, in certain configurations, the nanomaterial may beconverted into a nanopowder. A “wet chemistry” approach, giving thebenefit of low cost manufacturing and facile scalability, may be usedfor formation of such a nanopowder. In certain configurations, thenanomaterial may be formulated in a form convenient for use as solderingmaterial (e.g., in the form of a paste).

FIG. 2 illustrates exemplary nanoparticle structure in accordance withcertain aspects of the present disclosure. A nanoparticle 200 maycomprise a core 202, a shell layer 204 (typically an amorphous shelllayer) and a surfactant coating 206. The core 202 may be dense andcrystalline due to the high pressures of up to 5 GPa present during theformation process. The shell layer 204 may be amorphous, in which theatoms may be arranged more randomly, compared to the core 202, due tohigher radius of the nanoparticle's curvature. The amorphous arrangementin the shell layer 204 prevents the atoms from assuming a stable bulklattice arrangement and keeps them in a thermodynamically unstable statesuch that very little energy may cause the atoms in the shell layer 204to melt and flow. The arrangement of atoms in the shell layer 204 is, inpart, responsible for the reduced fusion temperature. In general, theremay not be a sharp boundary between the core 202 and the shell layer204.

The process described with reference to FIG. 1 may also be adapted forfabrication of nanoparticles for different metals such as aluminum. Forfabrication of aluminum nanomaterial, an aluminum precursor such astriethylaluminum (Al(C₂H₅)₃) and several alane derivatives may be usedbecause these chemicals are know to decompose at temperatures between160° C. to 100° C. in the gas phase. Furthermore, these precursorstypically decompose into the metal and a volatile byproduct. Thisbehavior can be represented by equation (7) below for triethylaluminum(AlR₃), heated to between 100° C. and 160° C. In solution, however, theprecursor is stabilized resulting in an observed decompositiontemperature above about 250° C.

AlR₃+heat+surfactants→nanoAl+3R(ethylene,propylene,hydrogen)  (7)

In a certain configuration of the present disclosure, aluminumnanoparticles can be produced by the reduction of for example analuminum halide precursor by activated sodium, as shown by equation (8)below:

AlCl₃+3Na*→3NaCl+nanoAl  (8)

Soldering material having a low fusion temperature may be advantageousin certain applications because low fusion temperature facilitates theuse of low cost packaging and board materials. In certainconfigurations, copper and aluminum nanomaterials may be mixed to obtaina soldering nanomaterial having a lower fusion temperature compared to acopper-only soldering nanomaterial.

In certain configurations of the present disclosure, a mixture of copperand aluminum nanomaterials may be made by fabricating coppernanomaterial and aluminum nanomaterial sequentially. Copper may bereduced first to form nanoparticles, followed by thermal decompositionof an aluminum precursor. The copper nanoparticles may function asnucleation sites and aluminum may deposit and coat the surface of thecopper nanoparticles resulting in a copper core 202 and an aluminumshell layer 204. The fusion temperature of the mixture may be based onthe ratio of copper/aluminum atoms in the shell layer 204.

Accordingly, in one aspect, a reduced fusion temperature solderingmaterial can be fabricated using copper and aluminum nanomaterials. Thefusion temperature of the soldering nanomaterial can be tailored viaalloying copper nanoparticles with aluminum. As the amount of aluminumused in relation to the amount of copper used in nanoparticle formation,the fusion temperature of the resulting nanomixture is closer to thefusion temperature of aluminum-only nanomaterial (approximately 130°C.).

In certain configurations of the present disclosure, the copper andaluminum nanoparticles can be made separately and then mixed in thedesired ratio to fabricate the final nanomaterial. In solderingnanomaterial created by mixing copper and aluminum nanomaterials, thepart of the mixture with lower melting point (copper or aluminum,depending on size of nanoparticles) will migrate first and function asglue bonding the other nanoparticles together. After soldering, thenanomaterial will typically convert to a metal matrix composite havingcores of original nanomaterial that are surrounded by thin layers theother nanomaterial. In certain configurations, the mixing of copper andaluminum nanoparticles can be advantageously used to fabricate pastescontaining about 30-50% nanoparticles (in the >0 and ≦20 nanometerrange) and the remaining 70-50% can be much larger in size, for example,micron size particles (e.g., 1 micron, 10 microns, 20 microns orlarger). The nanoparticles may function as a glue and may bond thelarger particles together and to the adjacent surfaces. In certainconfigurations, amounts less than about 30% may lead to increasinglyporous materials with less bonding and therefore reduced mechanicalintegrity.

Because the above described processes and selection of chemicalsfacilitate control over rate of nanoparticle formation (e.g., bycontrolling the mixing rate) and diameter distribution of the resultingnanoparticles (e.g., by choice of a surfactant), homogeneous andisotropic nanomaterial having approximately same fusion temperature astraditional lead solder can be fabricated, making it possible to use thenanomaterial as a drop-in replacement of lead solder used in thetraditional soldering equipment.

Practitioners of the art will appreciate that various processes ofcopper and aluminum nanomaterial formation described above may besynthesized using a single reaction apparatus, making it practicallyefficient and economical to fabricate soldering nanomaterial of adesired fusion temperature.

In certain configurations, depending on the desired fusion temperature,copper nanoparticles can be coated with a thin layer of aluminum (Al) tofurther reduce the fusion temperature of the resulting nanomaterial.Accordingly, in one aspect, a nanomaterial comprising coppernanoparticles coated with Al may be fabricated to achieve a fusiontemperature approximately equal to 130° C.

It will be appreciated by those skilled in the art that the high radiusof curvature of the fabricated nanoparticles prevents the surface layeratoms to be in a thermodynamically activated state requiring very littleenergy to start migrating and flowing. The atoms may quickly settle in athermodynamically stable lattice arrangement forming a solid bulkmaterial. The nanoparticle core may not take part in the formation ofthe lattice arrangement and the solid bulk material comprising coppernanocores surrounded by a thin layer of a copper lattice arrangementexhibits properties such as increased strength and fracture toughness.The lattice arrangement is retained to relative high temperatures beforegrain growth sets in, because the nanocores are stabilized by the matrixformed due to the lattice arrangement.

As described previously, in one aspect, the nanomaterial produced inaccordance with the present disclosure may be a drop-in replacement oflead based solder. No significant changes and capital investment may beneeded to adjust existing assembly lines. In another aspect, a lead-freesoldering material having a fusion temperature around 200° C.facilitates low cost packaging compared to a 260-300° C. fusiontemperature exhibited by SAC-based soldering material. Furthermore, theuse of copper helps with formation of strong joints enhanced fracturetoughness. Also, the use of such nanomaterial may minimize coefficientof thermal expansion (CTE) mismatch because vias, pins and bondingmaterial can be all copper based. Furthermore, unlike SAC material,copper nanomaterial does not exhibit whisker formation, therebyimproving circuit board life and reliability. The use of nanomaterialalso ensures very high uniformity of material, down to the nanoscale.Due to reduced fusion temperature, very rapid fusion process (<1 min)enabling a fast assembly process may be possible. In addition,copper-based nanomaterial offer the advantage of higher electrical andthermal conductivity compared to SAC based materials. In one aspect, dueto the use of copper in conjunction with nanoparticle technology jointsmay be made to be relatively insensitive to variations in composition,pad finishes, and process parameters, facilitating reliablemanufacturing under a large variety of conditions.

The use of organic surfactants, as described before, createsnanomaterial having organic surfactant coating 206 on the nanoparticles200, making the nanomaterial soluble in many organic solvents. Thisadvantageously allows the formulation of paint-like slurries and pastesfor spray-on, brush on, or other desired method of application. Suitablesolvents such as ethanol, isopropanol (IPA), acetone, toluene, as wellas additives such as paraffin wax and organic acids may be used for avariety of applications.

FIGS. 3A, 3B and 3C illustrate an example of soldering an electroniccomponent to a board (e.g., a PCB) using a nanomaterial, in accordancewith certain configurations of the present disclosure. In a typicalsoldering operation, an electronic component is attached to a board bysoldering contact points (e.g., pins, bond pads) of the component to theboard.

FIG. 3A shows a nanomaterial layer 304 comprising nanoparticles 200applied between an electronic component surface 302 and a board bond pad308. The nanoparticles 200 have a core 202, an amorphous shell layer 204and a surfactant coating 206, as previously discussed with respect toFIG. 2. While the nanoparticles 200 in FIG. 3A are shown to haveapproximately same diameter, in general, the nanomaterial layer 304 maycomprise of nanoparticles 200 having a range of diameters (e.g., between2 and 20 nanometers). The nanomaterial layer 304 is then heated to thefusion temperature of the nanomaterial. The nanolayer 304 may be heatedusing a laser, a rapid thermal annealing (RTA) system or other wellknown techniques .

FIG. 3B is a conceptual block diagram illustrating exemplary effect ofheating the nanomaterial layer 304. In the illustration, the surfactant206 has evaporated or melted away due to the elevated temperature,exposing the shell layer 204 and nanoparticle cores 202 in atransitional layer 306. Note that because the surfactant 206 hasevaporated, the component surface 302 and the board bond pad 308 now maybe physically closer to each other in comparison the configuration inFIG. 3A.

FIG. 3C shows exemplary configuration showing the component surface 302soldered to the board bond pad 308. In the illustrated configuration,the soldering layer 312 comprises a relatively homogenous conductivelayer 314 formed by fusion of the amorphous shell layers 206 of thenanoparticles 200 due to exposure to heat and nanoparticle cores 200embedded within the conductive layer 314. Because the shell layers 204has now turned into a homogenous conductive layer 314, the componentsurface 302 and the board bond pad 308 may further be closer to eachother than as shown in FIG. 3B. Application of heat using a traditionalheating system used for soldering thus results in a small, conductiveand relatively strong soldering joint between the component surface 302and the board bond pad 308.

An additional advantage of the soldering nanomaterial may be that thetotal soldering time (including heat application time) is relativelyshort (e.g., 1 to 2 minutes), since the nanoparticles 200 anneal veryrapidly. This is in part due to the nanoparticles 200 having anextremely small radius of curvature, forcing the atoms located in thatsurface shell into a thermodynamically unstable state. These unstableatoms liquefy rapidly at low temperature resulting in fusion of theunstable atoms into a thermodynamically more stable bulk lattice state

High performance semiconductor integrated circuits (ICs) and componentsfrequently have low thermal budgets. These devices can typicallywithstand fairly low exposure to higher temperatures before degradationsuch as dopant redistribution occurs. The low fusion temperature aspectof the nanomaterial fabricated according to certain aspects of thepresent disclosure render the use of such nanomaterial safe by reducingchances of inadvertent overheating of electronic components duringsoldering.

FIG. 4 is a flow chart of an exemplary process of soldering anelectronic component to a circuit board. At operation 402, solderingpaste comprising nanomaterial is applied between an electronic componentand a circuit board. The nanomaterial may comprise nanoparticles 200covered in shell layer 204 and surrounded by a surfactant coating 206,as described with respect to FIGS. 3A-3C. At operation 404, thesoldering paste is heated until the surfactant evaporates, enabling thenanoparticles 200 to fuse. The heating may be achieved by, for example,heating with a conventional oven or a more advanced laser. As describedwith respect to FIGS. 3B and 3C, the nanoparticles 200 fuse into a layer312 when the surfactant coating 206 evaporates (or melts) away allowingformation of a fusion layer 312. At operation 406, the heating solderingpaste is allowed to cool down. This results in a soldering joint beingformed between the electronic component and the circuit board (e.g.,conductive layer 314 in FIG. 3C).

In one aspect, the additional advantage of the nanomaterials of thepresent disclosure relates to the ability to “drop-in” the material intoexisting solder manufacturing processes. The nanomaterial can thereforebe readily incorporated into an existing production facility, enablingrapid technology transfer and product insertion. In one aspect,soldering material fabricated using the nanomaterials of the presentdisclosure are used to solder electronic components to a printedcircuit. In one aspect, a printed circuit may refer to a circuit board,circuit components, an assembly of circuits, an assembly of integratedcircuits and discrete components, or the like.

The mechanical joint properties of nanostructured copper andcopper/aluminum alloy phases are robust because they are relativelyinsensitive to variations in composition, pad finishes and processparameters. Such systems may be homogeneous and isotropic nanostructuredmaterial with fairly well-known constitutive relations. In certainconfigurations, it is possible to fabricate nanomaterial that is highlycompatible with current soldering methods, only minor changes such assolder pad sizing and pitch may have be made to current design tools andmethods.

Nanomaterial fabricated in accordance with the present disclosure can beadvantageously used in a variety of applications. In certainapplications, the nanomaterial generated in accordance with the presentdisclosure may be used in manufacturing of printed circuits for solarcells. A typical solar cell assembly comprises electronics on a flexiblesubstrate. Initially, a circuit may be prepared for printing on theflexible substrate, either by directly rendering the circuit on theflexible substrate or by preparing a photo mask for the circuit. Copperor aluminum nanomaterial fabricated into a paste may then be applied tothe flexible substrate and “etched” as needed. The “etching” operationmay be done as follows. First, a layer of the nanomaterial may bedeposited on a surface. Next, a desired conducting line of the printedcircuit (circuit lines) may be traced by a heat source such as a laser.The laser causes nanomaterial to heat up to its fusion temperature,thereby evaporating the surfactant on the traced line, leaving behindcopper a conductive line. It will be appreciated that the use of lasersto etch conductive lines facilitates precise and thin copper conductivelines useful for dense circuit designs. Next, after tracing with laser,the remaining nanomaterial is washed off by treating with a suitablesolvent. The washed off solution/mixture/paste may then be collected andre-used later to deposit a layer of copper nanomaterial on another boardbecause the solvent will contain dissolved nanomaterial.

In certain applications, the copper nanomaterial may be used to designcircuits using inkjet printing technology. The low fusion temperatureproperty of copper nanomaterial may be used in conjunction with theability to dissolve the copper nanomaterial to create a paste of desiredthickness. The paste may be used as the “ink” for the printer. Thecombined action of ink jet ejection of the “ink” along with heating tothe moderate fusion temperature may facilitate direct “printing” ofcircuits on a suitable printing material (e.g., a special paper or acircuit board).

The subject technology is illustrated, for example, according to variousaspects described below. Numbered clauses are provided below forconvenience. These are provided as examples, and do not limit thesubject technology.

In one aspect, a low melting point composition suitable for lead-freesoldering of electronic components may comprise a copper nanoparticlematerial, the nanoparticle material being dispersed with a solvent, thenanoparticle material having an average particle size equal to or lessthan 5 nanometers (Cu), where the low melting point composition iscapable of forming a soldering joint. The solvent may betetrahydrofuran.

In another aspect, a low melting point composition suitable forlead-free soldering of electronic components may comprise a aluminumnanoparticle material, the nanoparticle material being dispersed with asolvent, the nanoparticle material having an average particle size equalto or less than 10 nanometers (Al), where the low melting pointcomposition is capable of forming a soldering joint.

In yet another aspect, the low melting point composition may comprisealuminum and/or copper nanoparticles having 20 nanometer particle sizeor less (but greater than zero) in the 30-50% proportion, and theremaining 70-50% comprising larger particles which can be in the micronsize range (for example, 1 micron, 10 microns, 20 microns or larger).The smaller nanoparticles may function as a glue bonding the largerparticles together and to the adjacent surfaces. An increasing smalleramount of small nanoparticles less than 30% may lead to increasedporosity and loss of mechanical strength after fusion, which may bedesired for certain applications. A paste composition may furtherinclude triethanolamine and/or organic acids as flux from acetic acid tohexadecanoic acid.

Those of skill in the art would appreciate that the various illustrativeblocks, modules, elements, components, methods, and operations describedherein may be implemented as electronic hardware, computer software, orcombinations of both. For example, operation 110 may be implemented aselectronic hardware, computer software, or combinations of both. Toillustrate this interchangeability of hardware and software, variousillustrative blocks, modules, elements, components, methods, andalgorithms have been described above generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application.Various blocks may be arranged differently (e.g., arranged in adifferent order, or partitioned in a different way) all withoutdeparting from the scope of the subject technology. For example, thespecific orders of blocks 102 and 104 in the process illustrated in FIG.1 may be rearranged, and some or all of the operations may bepartitioned in a different way.

It is understood that the specific order or hierarchy of steps in theprocesses disclosed is an illustration of exemplary approaches. Basedupon design preferences, it is understood that the specific order orhierarchy of steps in the processes may be rearranged. Some of the stepsmay be performed simultaneously. The accompanying method claims presentelements of the various steps in a sample order, and are not meant to belimited to the specific order or hierarchy presented.

The previous description is provided to enable any person skilled in theart to practice the various aspects described herein. The previousdescription provides various examples of the subject technology, and thesubject technology is not limited to these examples. Variousmodifications to these aspects will be readily apparent to those skilledin the art, and the generic principles defined herein may be applied toother aspects. Thus, the claims are not intended to be limited to theaspects shown herein, but is to be accorded the full scope consistentwith the language claims, wherein reference to an element in thesingular is not intended to mean “one and only one” unless specificallyso stated, but rather “one or more.” Unless specifically statedotherwise, the term “some” refers to one or more. Pronouns in themasculine (e.g., his) include the feminine and neuter gender (e.g., herand its) and vice versa. Headings and subheadings, if any, are used forconvenience only and do not limit the invention.

Terms such as “top,” “bottom,” and the like as used, and blocks inillustrations as drawn on top or bottom of another blocks in thisdisclosure should be understood as referring to an arbitrary frame ofreference, rather than to the ordinary gravitational frame of reference.Thus, a top surface and a bottom surface may extend upwardly,downwardly, diagonally, or horizontally in a gravitational frame ofreference.

A phrase such as an “aspect” does not imply that such aspect isessential to the subject technology or that such aspect applies to allconfigurations of the subject technology. A disclosure relating to anaspect may apply to all configurations, or one or more configurations.An aspect may provide one or more examples. A phrase such as an aspectmay refer to one or more aspects and vice versa. A phrase such as a“configuration” does not imply that such configuration is essential tothe subject technology or that such configuration applies to allconfigurations of the subject technology. A disclosure relating to aconfiguration may apply to all configurations, or one or moreconfigurations. A configuration may provide one or more examples. Aphrase such a configuration may refer to one or more configurations andvice versa.

The word “exemplary” is used herein to mean “serving as an example orillustration.” Any aspect or design described herein as “exemplary” isnot necessarily to be construed as preferred or advantageous over otheraspects or designs.

All structural and functional equivalents to the elements of the variousaspects described throughout this disclosure that are known or latercome to be known to those of ordinary skill in the art are expresslyincorporated herein by reference and are intended to be encompassed bythe claims. Moreover, nothing disclosed herein is intended to bededicated to the public regardless of whether such disclosure isexplicitly recited in the claims. No claim element is to be construedunder the provisions of 35 U.S.C. §112, sixth paragraph, unless theelement is expressly recited using the phrase “means for” or, in thecase of a method claim, the element is recited using the phrase “stepfor.” Furthermore, to the extent that the term “include,” “have,” or thelike is used in the description or the claims, such term is intended tobe inclusive in a manner similar to the term “comprise” as “comprise” isinterpreted when employed as a transitional word in a claim.

1. A composition, comprising: metal nanoparticles, said metalnanoparticles having a diameter of 20 nanometers or less but greaterthan zero, said metal nanoparticles comprising a copper or aluminumnanocores, said composition having a fusion temperature of less thanabout 220° C.
 2. The composition of claim 1, comprising: an organicsurfactant coating said metal nanoparticles making the compositionsoluble in a solvent.
 3. A printed circuit comprising a conductive linehaving the composition of claim
 1. 4. A method of fabricating ananomaterial, comprising: preparing a solvent; adding a precursor toform a solution, the precursor comprising a metal, said metal being oneof aluminum or copper; adding a first surfactant to the solution; mixingin a reducing agent, at a mixing rate, with the solution until symptomsof nanoparticle formation occur in the solution; and adding a secondsurfactant to the solution to stop nanoparticle formation andagglomeration after the symptoms of nanoparticle formation occur.
 5. Themethod of claim 4, wherein the precursor comprises copper, and theoperation of preparing a solvent comprises preparing a solventcomprising tetrahydrofuran.
 6. The method of claim 4, wherein theprecursor comprises copper, and the operation of adding a precursorcomprises adding one or more of copper halide, copper nitrate, copperacetate, copper sulphate, copper formate and copper oxide.
 7. The methodof claim 4, wherein the precursor comprises copper, and the operation ofadding a first surfactant comprises adding one or more oftrioctylphosphine oxide, phosphine, a sulfate and a sulfonate.
 8. Themethod of claim 4, wherein the precursor comprises copper, and theoperation of mixing in a reducing agent comprises mixing in a reducingagent until the solution changes color.
 9. The method of claim 4,wherein the mixing rate has a value less than or equal to 1.5milliliters per minute.
 10. The method of claim 4, further comprising:drying the precursor to avoid oxidization due to exposure to moistureand air-oxygen.
 11. A method of soldering an electronic component to acircuit board, comprising: applying a soldering paste comprising ananomaterial between the electronic component and the circuit board, thenanomaterial comprising a plurality of nanoparticles surrounded by asurfactant; heating the soldering paste until the surfactant meltscausing the plurality of nanoparticles to fuse; and allowing the heatedsoldering paste to cool down to form a soldering joint between theelectronic component to the circuit board.
 12. The method of claim 11,wherein the operation of heating comprises heating using a laser.
 13. Acomposition, comprising: nanoparticles comprising a copper nanocore, anamorphous aluminum shell and an organic surfactant coating, saidnanoparticles having a diameter of 20 nanometers or less but greaterthan zero.
 14. A printed circuit comprising a conductive line having thecomposition of claim
 13. 15. The composition of claim 13, wherein theorganic surfactant coating comprises at least one of an amine and aphosphine.
 16. The composition of claim 13, further comprising one ormore of triethanolamine, acetic acid and hexadecanoic acid.
 17. A methodof manufacturing a printed circuit comprising: applying a metalnanomaterial comprising a surfactant to a surface; tracing, by a heatsource, a circuit line of the printed circuit on the surface, saidtracing causing the surfactant on the traced circuit line to evaporate;and treating the surface with a solvent to cause the untraced metalnanomaterial to wash off.
 18. The method of claim 17, furthercomprising: collecting the washed off solvent for a later use.
 19. Acomposition comprising copper or aluminum nanoparticles, wherein about30-50% of the copper or aluminum nanoparticles has a diameter of 20nanometers or less but greater than zero, and the remaining 70-50% ofthe copper or aluminum nanoparticles has a diameter greater than 20nanometers.
 20. The composition of claim 19, further comprising one ormore of triethanolamine, acetic acid and hexadecanoic acid.